EIT USING MAGNITUDE AND PHASE IN AN EXTENDED FREQUENCY-RANGE

The electrical impedance is a characteristic tissue property that can be used for imaging cross sections of the body. The full information contained in the complex tissue impedance can be utilized if not only the real part Re(Z) or the magnitude of the impedance but also the imaginary part Im(Z) or the phase is considered. Impedance measurements provide information about tissue structure, particularly extracellular space and cell membranes. Therefore, an electrical impedance tomograph was constructed which uses alternatively the real component, the imaginary component, the magnitude or the phase in an extended frequency range. The components are evaluated by digital correlation. The device allows state-different or frequency-different (almost static) imaging. 16 electrodes are used. Image reconstruction is arrived at by a back-projection algorithm. For frequency-different imaging the measured imaginary part values can be used after normalization (division by the measuring frequency); instead of the phase values, the quotients Im(Z)/Re(Z) are taken and divided by the actual frequency, representing time constants of the tissue. Frequency-different measurements on a tank filled with saline containing a metallic rod and an insulator show in a very illustrative manner the impedance of the metal/electrolyte boundary layer (phenomenon of electrode polarization). The first in vivo measurements are very promising, state-different as well as frequency-different images of the human thorax represent, for example, the lungs with higher contrast using the phase than using the magnitude. Frequency-different (static) images of a cross section of the lower part of the leg exhibit, using the time constants tau, an unusually distinct representation of the muscles in contrast to images based on the real part or magnitude.